1. Technical Field
The disclosure relates in general to a liquid crystal display and a pixel array substrate thereof, and more particularly, to a liquid crystal display capable of preventing flickering, and a pixel array substrate thereof.
2. Description of the Related Art
FIG. 1 is a schematic illustration showing a circuit of a conventional liquid crystal display (LCD) with thin film transistors (TFTs). In FIG. 1, thin film transistors TFT10A, TFT10B, TFT10C . . . , which are contained in sub-pixels SP10A, SP10B, SP10C . . . on the same row, are driven by the same scan line S10. One main pixel is usually composed of three sub-pixels. For example, one main pixel capable of displaying any desired color is usually composed of red, green and blue sub-pixels. When the scan line S10 provides the sufficient turn-on voltage, the thin film transistors TFT10A, TFT10B, TFT10C . . . connected to the scan line S10 are turned on such that the data (voltage levels) carried by data lines D10 can be written into the sub-pixels SP10A, SP10B, SP10C . . . . After the above-mentioned writing operation is finished, the thin film transistors TFT10A, TFT10B, TFT10C . . . are turned off, and the voltage level of the pixel electrode in each of the sub-pixels SP10A, SP10B, SP10C . . . is held through a liquid crystal capacitor CLC and a pixel storage capacitor CST.
However, when the thin film transistors TFT10A, TFT10B, TFT10C . . . are turned off, the voltage level of the pixel electrode in each of the sub-pixels SP10A, SP10B, SP10C . . . tends to be influenced by the changes of other ambient voltages and thus fluctuates. This voltage fluctuation amount is referred to as a “feed-through voltage”, VFD, which may be represented in the following Equation (1):VFD=[CGD/(CLC+CST+CGD)]×Δ VG   (1)where CLC represents the liquid crystal capacitor, CST represents the pixel storage capacitor, CGD represents the capacitor between the gate and the drain of the thin film transistor, and Δ VG represents the voltage difference between the voltages of the scan line when the thin film transistor is turned on and off.
According to the operational principle of the known liquid crystal display, rotation angles of liquid crystal molecules vary with changes in the intensity of the electric field applied to the liquid crystal molecules so that various gray levels may be displayed. The intensity of the electric field applied to the liquid crystal molecules is determined according to the voltage difference between the pixel electrode of each sub-pixel and the common electrode. Thus, when the voltage level of the pixel electrode of the sub-pixel is influenced by the feed-through voltage VFD and thus changed, there is a deviation of the gray level actually displayed by the sub-pixel from the desired gray level to be displayed, thereby flickering occurs and the display quality of the known liquid crystal display is influenced. Fortunately, the feed-through voltages VFD of one sub-pixel only slightly differ from another, therefore, this drawback may be solved by performing overall compensations on the driving voltage levels of the sub-pixels when the display signals are inputted.
Generally speaking, many spacers are usually disposed between a pixel array substrate (i.e., the substrate that contains TFTs) and an opposing substrate, which are paired with liquid crystal material sandwiched in between to constitute a liquid crystal display panel, so as to maintain the desired cell gap between the substrates. The spacers usually only need to be disposed over the pixel storage capacitor of one sub-pixel of every pixel, and are usually made of photosensitive resin and formed on the surface of the opposing substrate by way of photo-lithography. Such spacers are also referred to as photo spacers (PSs).
FIG. 2 is a top view showing a pixel array of a TFT LCD known to the inventor(s). As shown in FIG. 2, in the pixel array composed of sub-pixels SP20A, SP20B, SP20C for each pixel, there is provided a common line C20 penetrating through the sub-pixels. The common line C20 and a capacitor electrode, which is electrically connected to the pixel electrode E20A, E20B, E20C of each sub-pixel, are capacitively coupled to each other and function as a pixel storage capacitor CST while a common voltage or a desired level voltage is written into the common line C20. Recent developments in mobile communications emphasize customer demands for small-size, high-resolution LCD products. Thus, the size of each sub-pixel has to be reduced according to this trend. However, the photo spacer, e.g., photo spacer 10 located in sub-pixel SP20C of the pixel shown in FIG. 2, still has to maintain the predetermined width or diameter to ensure its function of supporting and maintaining the cell gap between the opposing substrates. Therefore, the capacitor electrode E20C, which supports the photo spacer 10 and is for shielding display defects associated with disclination caused by the disordered liquid crystal molecules around the photo spacer 10, needs to have a predetermined size that is usually greater than the width of the common line C20 in a high-resolution, small-size LCD product. Thus, the widths of the common line C20 of the sub-pixel SP20C and the capacitor electrode E20C have to be properly enlarged so that the spacer 10 can be disposed thereon. However, in order to unify the capacitances of the storage capacitors CST of the sub-pixels and thus maintain the relationship among the feed-through voltages of the sub-pixels, i.e. VFD-A≈VFD-B≈VFD-C, and further to avoid flickering by driving voltage compensation, even in the other two sub-pixels, i.e., SP20A, SP20B, where no spacer is disposed, the areas of two capacitor electrodes E20A and E20B also have to be correspondingly enlarged. Consequently, in the sub-pixels SP20A, SP20B and SP20C, the relationship of the feed-through voltages VFD-A≈VFD-B≈VFD-C can be maintained by obtaining CST-A=CST-B=CST-C, with, however, sacrifices in the aperture ratios of the other sub-pixels (SP20A, SP20B) where no spacer is disposed. Thus, the overall display brightness is reduced, and the display quality and the product performance are significantly influenced.